The present invention relates to the preparation of optical quality rigid-chain polymer thin films.
In the past several years, nonlinear optics has emerged as a new frontier of scientific research for technological development of photonics by which information may be acquired, stored, processed and transmitted through photons. Organic polymers typically have the desirable mechanical integrity, flexibility, light weight, low linear optical loss, and low temperature solution or melt processibility. Interest in highly conjugated polymers for nonlinear optical applications stems from the potential combination of the typical polymer properties with the unique electronic and optical characteristics of conjugated polymers. Their .pi.-electron delocalization can lead to large optical nonlinearities and their femtosecond response time is by far the fastest compared to inorganic materials and multiple quantum wells.
One group of polymers of particular interest are the para-ordered heterocyclic polymers. This group, commonly referred to as rigid-rod or rigid-chain polymers, has repeating units of the general formula --(--Z--Ar--)--, wherein Z is benzobisazole group and Ar is a para-oriented aromatic moiety, such as 1,4-phenylene, 4,4'-biphenylene, 4,4'-diphenylsulfide, 4,4'-diphenylether or the like. Thus, the group includes poly(p-phenylene benzobisoxazole) (PBO), poly(p-phenylene benzibisthiazole) (PBT) and poly(p-phenylene benzobisimidazole) (PBI) polymers and co-polymers, as well as substituted derivatives thereof.
The preparation and processing of rigid-rod polymers and co-polymers is well known in the art. They are commonly prepared by the condensation of at least one di- or tetra-amino monomer with at least one para-oriented dicarboxylic acid monomer in a strong acid such as polyphosphoric acid (PPA). The polymer or co-polymer is commonly recovered from the reaction mixture by precipitation in water. The polymer or co-polymer can be formed into desirable shapes by directly extruding or spin-casting the PPA mixture into water. Alternatively, the polymer or co-polymer can be precipitated into water, washed, dried and then taken up in a strong acid, such as methanesulfonic acid (MSA), then extruded, cast or spin-cast into water.
Rigid-rod polymers and co-polymers generally have very high thermal stability, high mechanical strength and low solubility in ordinary solvents. These polymers and co-polymers generally have melting temperatures higher than their thermal degradation temperatures; thus, they cannot be processed by melt-processing techniques. As noted previously, they are normally processed into desirable shapes by extrusion, casting or spin-casting from a strong acid solution, such as from a methanesulfonic acid solution. Optical quality thin films of PBT have been fabricated by coagulation from solutions in MSA or PPA and by spin-casting from organic solvents with Lewis acids. Over the years, the third-order nonlinear optical susceptibility, .chi..sup.(3), of PBT thin films showed significant increase from 6.times.10.sup.-12 to 4.5.times.10.sup.-10 esu. In the conventional extrusion and coagulation process used for making rigid-rod films, the films normally show four types of defects: (a) large voids of about 20 .mu.m size, (b) small particles of about 1 .mu.m size, (c) lines parallel to the direction of extrusion and (d) lines transverse to the direction of extrusion. These defects are believed to significantly contribute to the optical loss of the rigid-rod films.
Helminiak et al, U.S. Pat. No. 4,377,546, issued Mar. 22, 1983, disclose a method for preparing a molecular composite film comprising the steps of forming a solvent mixture of methanesulfonic acid and a polymer mixture of rod-like, aromatic heterocyclic polymer and an amorphous, coil-like, aromatic heterocyclic polymer, casting the solvent mixture and exposing the resulting polymer layer to water vapor for a period of time sufficient to form a precipitated film. When used with a solvent mixture of methanesulfonic acid and a rod-like, aromatic heterocyclic polymer, this method provides films free of large voids. However, the resulting films had very poor optical transparency.
Accordingly, it is an object of this invention to provide a method for preparing optical quality, thin films of rigid-rod polymers and co-polymers.
Other objects and advantages of the present invention will be apparent to those skilled in the art.